The Savonius turbine, Darrieus turbine and the cup anenometer are examples of typical cross-flow wind turbines. A substantial advantage of these turbines is that their vertical axis of rotation makes them insensitive to wind direction. However, as these turbines have blades of fixed orientation fluid flow through the rotor is not always optimal. The performance of these types of turbines at `off-design` tip-speed ratios is therefore poor. (Tip-speed ratio is defined as the ratio between the blade velocity relative to the ground and the velocity of the free stream relative to the ground i.e. .OMEGA.R/V.sub.1). In particular, the Savonius and cup anenometer type turbines suffer large losses and only perform satisfactorily at low tip-speed ratios typically up to about 1.5. The Darrieus turbine on the other hand operates at higher tip-speed ratios typically 3 to 7, but the range of tip-speed ratios for effective operation is narrow and the turbine is not self-starting.